Description

Leptin (LEP, OB, OBS), a circulating adipokine, and its receptor LEPR (DB, OBR) control food intake and energy balance and are implicated in obesity-related diseases (recently reviewed in Amitani et al. 2013, Dunmore and Brown 2013, Cottrell and Mercer 2012, La Cava 2012, Marroqui et al. 2012, Paz-Filho et al. 2012, Denver et al. 2011, Lee 2011, Marino et al. 2011, Morton and Schwartz 2011, Scherer and Buettner 2011, Shan and Yeo 2011, Wauman and Tavernier 2011, Dardeno et al. 2010, Bjorbaek 2009, Morris and Rui 2009, Myers et al. 2008), including cancer (Guo et al. 2012), inflammation (Newman and Gonzalez-Perez 2013, Iikuni et al. 2008), and angiogenesis (Gonzalez-Perez et al. 2013).The identification of spontaneous mutations in the leptin gene (ob or LEP) and the leptin receptor gene (Ob-R, db or LEPR) genes in mice opened up a new field in obesity research. Leptin was discovered as the product of the gene affected by the ob (obesity) mutation, which causes obesity in mice. Likewise LEPR is the product of the gene affected by the db (diabetic) mutation. Leptin binding to LEPR induces canonical (JAK2/STATs; MAPK/ERK 1/2, PI-3K/AKT) and non-canonical signaling pathways (PKC, JNK, p38 MAPK and AMPK) in diverse cell types. The binding of leptin to the long isoform of LEPR (OB-Rl) initiates a phosphorylation cascade that results in transcriptional activation of target genes by STAT5 and STAT3 and activation of the PI3K pathway(not shown here), the MAPK/ERK pathway, and the mTOR/S6K pathway. Shorter LEPR isoforms with truncated intracellular domains are unable to activate the STAT pathway, but can transduce signals by way of activation of JAK2, IRS-1 or ERKs, including MAPKs.LEPR is constitutively bound to the JAK2 kinase. Binding of LEP to LEPR causes a conformational change in LEPR that activates JAK2 autophosphorylation followed by phosphorylation of LEPR by JAK2. Phosphorylated LEPR binds STAT3, STAT5, and SHP2 which are then phosphorylated by JAK2. Phosphorylated JAK2 binds SH2B1 which then binds IRS1/2, resulting in phosphorylation of IRS1/2 by JAK2. Phosphorylated STAT3 and STAT5 dimerize and translocate to the nucleus where they activate transcription of target genes (Jovanovic et al. 2010). SHP2 activates the MAPK pathway. IRS1/2 activate the PI3K/AKT pathway which may be the activator of mTOR/S6K.Several isoforms of LEPR have been identified (reviewed in Gorska et al. 2010). The long isoform (LEPRb, OBRb) is expressed in the hypothalamus and all types of immune cells. It is the only isoform known to fully activate signaling pathways in response to leptin. Shorter isoforms (LEPRa, LEPRc, LEPRd, and a soluble isoform LEPRe) are able to interact with JAK kinases and activate other pathways, however their roles in energy homeostasis are not fully characterized.
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The RAS-RAF-MEK-ERK pathway regulates processes such as proliferation, differentiation, survival, senescence and cell motility in response to growth factors, hormones and cytokines, among others. Binding of these stimuli to receptors in the plasma membrane promotes the GEF-mediated activation of RAS at the plasma membrane and initiates the three-tiered kinase cascade of the conventional MAPK cascades. GTP-bound RAS recruits RAF (the MAPK kinase kinase), and promotes its dimerization and activation (reviewed in Cseh et al, 2014; Roskoski, 2010; McKay and Morrison, 2007; Wellbrock et al, 2004). Activated RAF phosphorylates the MAPK kinase proteins MEK1 and MEK2 (also known as MAP2K1 and MAP2K2), which in turn phophorylate the proline-directed kinases ERK1 and 2 (also known as MAPK3 and MAPK1) (reviewed in Roskoski, 2012a, b; Kryiakis and Avruch, 2012). Activated ERK proteins may undergo dimerization and have identified targets in both the nucleus and the cytosol; consistent with this, a proportion of activated ERK protein relocalizes to the nucleus in response to stimuli (reviewed in Roskoski 2012b; Turjanski et al, 2007; Plotnikov et al, 2010; Cargnello et al, 2011). Although initially seen as a linear cascade originating at the plasma membrane and culminating in the nucleus, the RAS/RAF MAPK cascade is now also known to be activated from various intracellular location. Temporal and spatial specificity of the cascade is achieved in part through the interaction of pathway components with numerous scaffolding proteins (reviewed in McKay and Morrison, 2007; Brown and Sacks, 2009). The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).

As inferred from mouse, binding of Leptin (LEP) to the Leptin receptor (LEPR) causes a conformational change in LEPR that activates autophosphorylation of JAK2 at multiple tyrosine residues. Phosphorylated JAK2 has much higher kinase activity than unphosphorylated JAK2.

Analysis of a structural model of the Leptin-LEPR complex using as a basis the complex formed by granulocyte-colony stimulator factor (GCSF) and its receptor G-CSF R (Hiroike et al., 2000) suggested that helices I and III of the human leptin structure were likely sites of interaction with the cytokine binding domain of leptin receptor (Gonzalez and Leavis, 2003). It is believed that the Leptin receptor (LEPR) is a dimer constitutively bound in a complex with JAK2 kinase (Couturier and Jockers 2003). It has been proposed that one molecule of Leptin binds each monomer of LEPR (Luoh et al. 1997, Mistrik et al. 2004), however these suggestions need further proof becasue the structure of the Leptin:LEPR complex has not yet been solved.

SHP2 (PTPN11) interacts with phosphotyrosine-986 of the phosphorylated Leptin receptor (LEPR) (Carpenter et al. 1998). The corresponding site in mouse is phosphotyrosine-985 and in rat phosphotyrosine-986.SHP2 and SOCS3 compete for the same binding site on LEPR. SHP2 activates MAPK signaling, probably by recruiting GRB2:SOS which activates RAS.

STAT3 binds phosphotyrosine-1141 of the C-terminal, cytoplasmic region of LEPR (Bjorbaek et al. 1997). Only the long isoform of LEPR has tyrosine-1141 and consequently only the long isoform of LEPR activates STAT3. Short isoforms of LEPR exist but their function is uncertain. Shorter LEPR isoforms bind JAK2 and can signal through IRS-1 or ERKs, including MAPKs (Bjorbaek et al. 1997).

As inferred from mouse, SOCS3 binds LEPR at phosphotyrosine-986 and phosphotyrosine-1079. SOCS3 competes with SHP2 (PTPN11) for phosphotyrosine-986 and with STAT5 for phosphotyrosine-1079. SOCS3 expression is upregulated by leptin and SOCS3 downregulates prolonged leptin signaling, providing a feedback loop to limit leptin's action.

As inferred from mouse, both non-phosphorylated and phosphorylated STAT3 can form dimers and enter the nucleus. Phosphorylation of STAT3 appears to change the equilibrium between these states, causing accumulation of phosphorylated STAT3 in the nucleus. Phosphorylated STAT3 dimers also activate transcription more efficiently.

Phosphorylated STAT5A and STAT5B form homodimers and heterodimers in the cytosol (Gaffen et al. 1996, Rosenthal et al. 1997, also inferred from mouse homologs). Phosphorylation of a critical tyrosine residue in the SH domain (Y694 in STAT5A and Y699 in STAT5B) and intramolecular interactions between hydrophobic residues in the SH domain are required for dimerization (inferred from mouse homologs).